1. Field of the Invention
The present invention relates generally to an uplink power control method for a mobile communication system and, more particularly, to a per-layer and per-antenna uplink power control method for Multiple-Input Multiple-Output (MIMO) transmission in a Long Term Evolution-Advanced (LTE-A) system.
2. Description of the Related Art
Recently, a great deal of research has been conducted relating to the use of Orthogonal Frequency Division Multiple Access (OFDMA) and Single Carrier Frequency Division Multiple Access (SC-FDMA) as schemes for high-speed data transmission over a radio channel. In LTE, a next generation mobile communication system, OFDM is adopted in downlink transmission while SC-FDMA is adopted in uplink transmission.
Since it is well known that OFDMA has a high Peak-to-Average Power Ratio (PAPR), a large back-off is required for the input to the power amplifier to avoid nonlinear signal distortion, which lowers the maximum transmit power, resulting in low power efficiency. The back-off sets the maximum transmit power to a level lower than the maximum power of the power amplifier to ensure the linearity of the transmit signal. For example, when the maximum power of the power amplifier is 23 dBm and the back-off is 3 dBm, the maximum transmit power is restricted to 20 dBm.
OFDMA has no significant drawbacks as a downlink multiplexing technology because the transmitter is located in a base station that has no power shortage problem. However, OFDMA has significant drawbacks as an uplink multiplexing technology, because the transmitter of the user equipment has severe power limitations. The transmit power constraint of the user equipment may cause a reduction of service coverage of the base station. In order to overcome this problem, the LTE, as the fourth generation (4G) mobile communication standard of the 3rd Generation Partnership Project (3GPP), has adopted SC-FDMA as uplink multiplexing scheme.
With the advance of radio communication technologies that provide diverse multimedia service in recent advanced radio communication environments, a high-speed data transmission technique is required to support high quality multimedia services. In order to meet the requirements for high-speed data transmission, diverse research is being conducted and MIMO is one of the techniques upon which much of the research is focused.
MIMO employs multiple antennas to increase channel capacity within given frequency resource limitations. MIMO can produce a channel capacity that is proportional to the number of antennas in a scattering environment. In order to improve the data transmit efficiency of the MIMO technique, the transmit data is coded before transmission, which is commonly referred to as precoding. A precoding rule is defined with a matrix, i.e. a precoding matrix, and a set of precoding matrices is referred to as codebook. In LTE-Advanced (LTE-A), a precoding matrix-based MIMO is one of the key techniques for performance enhancement in uplink transmission in both single-user and multiuser environments.
In uplink of LTE, event-triggered power control is used for Physical Uplink Shared Channel (PUSCH). This means that there is no need to transmit the Transmit Power Control (TPC) periodically.
The PUSCH transmit power calculated for an ith subframe PPUSCH(i) can be expressed by Equation (1):PPUSCH(i)=min {PCMAX,10 log10(MPUSCH(i))+PO—PUSCH(j)+α(j)·PL+ΔTF(i)+f(i)}[dBm]  (1)where PCMAX denotes a maximum transmit power according to the power class of the User Equipment (UE). MPUSCH(i) denotes the PUSCH resource allocated in the ith subframe and is expressed by a number of Resource Blocks (RBs). The transmit power of the UE increases in proportion to MPUSCH(i). PL denotes a downlink Path Loss measured by the UE. The scaling factor α(j) is determined by a higher layer in consideration of the PL between uplink and downlink channels for establishing a cell. PO—PUSCH can be expressed by Equation (2):PO—PUSCH(j)=PO—NOMINAL—PUSCH(j)+PO—UE—PUSCH(j)  (2)where PO—NoMINAL—PUSCH(j) denotes a cell-specific parameter signaled by higher layer. PO—UE—PUSCH(j) denotes a UE-specific parameter transmitted by Radio Resource Control (RRC) signaling. The Modulation and Coding Scheme (MCS) or Transport Format (TF) compensation parameter ΔTF(i) is defined by Equation (3):
                                          Δ            TF                    ⁡                      (            i            )                          =                  {                                                                      10                  ⁢                                                            log                      10                                        ⁡                                          (                                                                        2                                                                                    MPR                              ⁡                                                              (                                i                                )                                                                                      ·                                                          K                              S                                                                                                      -                        1                                            )                                                                                                                                        for                    ⁢                                                                                  ⁢                                          K                      S                                                        =                  1.25                                                                                    0                                                                                  for                    ⁢                                                                                                              ⁢                                                                                                            ⁢                                          K                      S                                                        =                  0                                                                                        (        3        )            where MPR(i) is calculated by Equation (4):
                              MPR          ⁡                      (            i            )                          =                              TBS            ⁡                          (              i              )                                                                                            M                  PUSCH                                ⁡                                  (                  i                  )                                            ·                              N                SC                RB                            ·              2                        ⁢                          N              Symb              UL                                                          (        4        )            where TBS(i) denotes a transport block size in the ith subframe. In Equation (4), the denominator MPUSCH(i)·NSCRB·2NSymbUL is the number of Resource Elements (REs) in the subframe. Specifically, the MPR(i) obtained by Equation (4) refers to the amount of information bits per RE. If KS=0, MPR(i)=0, and MCS compensation is not considered. If KS=1.25, only 80% of uplink channel
  (            1              K        s              =    0.8    )is MCS-compensated. Instantaneous adaptation of PUSCH power control can be expressed by f(i) in Equation (5):f(i)=f(i−1)+δPUSCH(i−KPUSCH)  (5)where δPUSCH, denotes a UE-specific parameter included in the PDCCH transmitted from the base station to the UE and is referred to as TPC value. In δPUSCH(i−KPUSCH), KPUSCH denotes the time difference between for receipt of δPUSCH value and application of δPUSCH to the transmission subframe of the UE. The δPUSCHdB accumulation value in DCI format 0 is carried by PDCCH is [−1, 0, 1, 3]. The δPUSCHdB accumulation value in DCI format 3/3A is carried by PDCCH is [−1, 0, 1, 3].
In addition to the method for accumulating the δPUSCH value as in Equation (5), an absolute value of δPUSCH can be used as shown in Equation (6). In this case, the absolute value of δPUSCH in DCI format 0 is carried by PDCCH is [−4, −1, 1, and 4].f(i)=δPUSCH(i−KPUSCH)  (6)
As described above, it is difficult to apply the power control method for an LTE UE, which transmits a single codeword through a single antenna to the MIMO transmit UE, which transmits multiple codewords through multiple antennas on multiple layers without modification.